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The sponge matrix allograft model made possible the growth of an AKR lymphoma in a certain percentage of BALB/c mice making progressors (tumor-bearing) and regressors (tumor-rejecting) simultaneously available. Mice bearing either an AKR kidney allograft or a sponge alone were used as controls. The cell population infiltrating the sponge was evaluated 2, 5, 10, 15, and 21 days after subcutaneous implantation. It consisted of macrophages, lymphocytes, and neutrophils. There was no difference between groups on Day 2 and Day 5. From Day 10 onwards, tumor growth was evident with a clear cut separation between progressors and regressors. The latter behaved like the two control groups except for a significant increase in lymphocytes on Day 21. The progressors showed a striking increase in total cell count from Day 10 onwards. Macrophages were the major population with a maximum value of 201 X 10(6) as compared to 8 X 10(6) in the regressors; their phagocytic and lysosomal activity remained similar in all groups. The lymphocytes showed no variation in absolute numbers but, because of the high cell count in progressors, their ratio to macrophages reached 1:50 on Day 10, making up only 2% of the cell population. Neutrophils were significantly increased in progressors as compared with the other groups. It can be concluded that in the presence of a foreign body reaction a marked influx of macrophages accompanies allogeneic tumor growth.